U.S. Pat. No. 5,480,217 issued to Ohtsu on Jan. 2, 1996 discloses a known anti-lock braking system (ABS) for automobiles. As is so in this known ABS, it is the conventional practice to compare a wheel speed assigned to a road wheel to be controlled with a reference value, and to regulate the brake pressure in response to the result of this comparison. This wheel speed may not necessarily be equal to an actual wheel speed of the road wheel to be controlled. For making a clear distinction from the actual wheel speed, the above-mentioned wheel speed that is compared with the reference value is hereinafter named as the term "controlled wheel speed." The controlled wheel speeds for all of the road wheels are determined. These determined controlled wheel speeds are used to give a pseudo-vehicle speed that approximates an actual vehicle travelling speed. This pseudo-vehicle speed is used in determining the above-mentioned reference value. This reference value is indicative of a wheel speed, which is slightly lower than a wheel speed that corresponds to the pseudo-vehicle speed, to be accomplished for establishing the optimum braking state at the vehicle speed. The controlled wheel speed for the road wheel to be controlled is compared with the reference value. Immediately after the controlled wheel speed has exceeded the reference value, the brake pressure within the wheel cylinder is decreased, thereby suppressing the tendency of the road wheel to lock.
With regard to the manner of determining the controlled wheel speeds, there are three techniques for determining controlled wheel speeds for rear road wheels. They include a rear wheel select-low control, a rear wheel independent control, and a diagonal select-low control.
According to the rear wheel select-low control, a lower one of actual wheel speeds VRR and VRL of rear right and rear left road wheels is selected and used as a controlled wheel speed VwR for the rear road wheels.
According to the rear wheel independent control, an actual wheel speed VRR is used as a controlled wheel speed VwRR for a rear right road wheel, while an actual wheel speed VRL is used as a controlled wheel speed VwRL for a rear left road wheel.
According to the diagonal select-low control, a lower one of an actual wheel speed of a rear road wheel to be controlled and an actual wheel speed of the nonadjacent diagonally arranged front road wheel is selected and used as a controlled wheel speed for the rear road wheel.
The rear wheel select-low control has a shortcoming that appears during braking in cornering state. FIG. 7(a) illustrates, in vector, braking forces applied to each road wheels of an automobile, with an ABS employing the rear wheel select-low control, during braking in a left turn cornering state. In the cornering state illustrated in FIG. 7(a), the rear left wheel is subject less heavier load than load on the rear right wheel, so that the rear left wheel rotates at wheel speed VRL, which is lower than wheel speed VRR of the rear right wheel. In this case, according to the select-low logic, the rear left wheel speed VRL is used as a controlled wheel speed VwR for the rear wheels. The controlled wheel speed VwR that is equal to the rear left wheel speed VRL is used in controlling brake pressure within wheel cylinders of both of the rear wheels. Thus, brake force applied to the rear right wheel is the same as brake force applied to the rear left wheel as illustrated in vectors in FIG. 7(a). This braking force is high enough for allowing the rear left wheel to rotate at the reference value, which is indicative of a wheel speed that is slightly lower than the pseudo-vehicle speed. But it is not high enough to allow the rear right wheel to rotate at the reference value, thereby allowing the rear right wheel to rotate above the wheel speed that is indicated by the reference value. Thus, the rear wheel select-low control may hamper accomplishment of a reduction in braking distance during braking in cornering state.
The rear wheel independent control is free from the above-mentioned shortcoming of the rear wheel select-low control.
However, the rear wheel independent control exhibits a shortcoming during travelling of the automobile with road wheels on one side in contact with low .mu. road surface and road wheels on the opposite side in contact with high .mu. road surface. FIG. 7(b) illustrates, in vector, braking forces applied to each road wheels of an automobile, with an ABS employing the independent control, during braking when the vehicle is travelling with the left wheels on low .mu. road surface and the right wheels on high .mu. road surface. In this case, the rear right wheel has fewer tendencies to slip than the rear left wheel has. Thus, braking the rear road wheels results in applying relatively high braking force to the rear right wheel as compared to braking force applied to the rear left wheel. Application of such relatively high braking force to the rear right wheel in contact with high .mu. road surface will cause a reduction in cornering force Sf. The reduction in cornering force Sf may hamper accomplishment of stable behavior of the vehicle.
The diagonal select-low control is free from the above-mentioned shortcoming of the rear wheel independent control. FIG. 7(c) illustrates, in vector, braking forces applied to each road wheels of an automobile, with an ABS employing the diagonal select-low control, during braking when the vehicle is travelling with its left wheels on low .mu. road surface and its right wheels on high .mu. road surface. According to the diagonal select-low control, a lower one of an actual wheel speed of a rear road wheel to be controlled and an actual wheel speed of the nonadjacent diagonally arranged front road wheel is selected and used as a controlled wheel speed for the rear road wheel. Since the front left wheel speed VFL is lower than the rear right road wheel VRR, the front left wheel speed VFL is used as a controlled wheel speed VwRR for the rear right wheel. Thus, the braking force applied to the rear right wheel is suppressed as compared to the braking force applied to the rear right wheel illustrated in FIG. 7(b), thereby inducing a relatively large cornering force Sf as compared to that illustrated in FIG. 7(b). The cornering force Sf increases as the braking force decreases, and the former decreases as the latter increases.
Further, the diagonal select-low control is free from the shortcoming, during braking in cornering state, of the rear wheel select-low control. FIG. 7(d) illustrates, in vector, braking forces applied to each road wheels of an automobile, with an ABS employing the diagonal select-low control, during braking in a left turn cornering state. According to the diagonal select-low control, a lower one of the rear right wheel speed VRR and the front left wheel speed VFL is used as the controlled wheel speed VwRR for the rear road wheel. Thus, the rear left wheel speed VRL is no longer used as the controlled wheel speed VwRR. As VFL is higher than VRL during braking in left turn cornering state, sufficiently high braking force is ensured for the rear right wheel.
However, the diagonal select-low control shows a shortcoming during braking when the automobile is travelling in a straightforward direction. FIG. 7(e) illustrates braking forces, in vector, applied to each road wheels of an automobile, with an ABS employing the diagonal select-low control, during travelling in a straightforward direction. In this state, if the front right and left wheels are subject to slip during braking, their wheel speeds VFR and VFL decrease. According to the diagonal select-low control, the front right wheel speed VFR and front left wheel speed VFL are used as the rear right controlled wheel speed VwRR and rear left controlled wheel speed VwRL, respectively. Thus, braking forces applied to the rear road wheels may drop below a satisfactory level.
An object of the present invention is to eliminate the shortcoming of the diagonal select-low control, thereby accomplishing sufficiently high braking forces applied to each road wheels during braking when an automobile is travelling in a straightforward direction, while retaining all of the merits of the diagonal select-low control.